The present invention is particularly intended for offshore oil prospecting.
In the sense of the invention flexible line is understood to mean flexible pipes, in particular flexible tubular pipes for transporting fluids such as hydrocarbons, composite bundles, umbilical cords, or electrical cables. Structure is understood to mean any fixed or moving structure which can be used in the offshore field, such as a fixed rigid Jacket structure, an oscillating structure rigidly attached to the sea bed or a floating structure such as a naval surface support, a semisubmersible platform, a floating vertical column which is anchored, connected to an oil tanker for storage/separation which is anchored to it, a buoy, etc.
More precisely, the invention relates to a device comprising a curvature limiter engaged on the said flexible line and secured to fixing means able to interact with fixing means secured to the said hollow rigid member for fastening the curvature limiter to the said structure.
Curvature limiter is understood to be any type of device making it possible to impose upon the flexible line a radius of curvature which is greater than a minimum radius of curvature in a zone where the flexible line is subjected to bending, particularly repeated bending, risking damaging it.
The curvature limiter may be of any known type, for example, made up of articulated rigid elements called vertebrae, or preferably in the form of a stiffener.
Known stiffeners are generally moulded blocks made from plastic such as a polyurethane, of at least partially frustoconical shape, installed on the flexible line and fastened to a fastening support mounted on and/or around the flexible line.
Fastening in the sense of the invention, is understood to be a rigid mechanical link, at least in bending and with respect to lateral translations, between the curvature limiter and its fastening support, that is to say a link such that the fastening support blocks the angular displacements (curvature about any axis perpendicular to the axis of the flexible line) and the transverse displacements of the curvature limiter and, consequently of the flexible line, the values associated with bending torques and bearing forces being wholly withstood by the fastening support. As a variant, the said fastening may be an integral fastening, also ensuring the blocking of the axial displacements and the torsion forces about the longitudinal axis of the flexible pipe.
The hollow rigid member is a part of the structure or a member securely fastened onto the structure, such as a platform, to which one end of the flexible pipe is connected, the said hollow rigid member comprising a central opening for the passage of the flexible line and constituting the last bearing point of the flexible line on the support platform, like a hawsepipe. The hollow rigid member which constitutes the fastening support for the curvature limiter is commonly an end section of a tube guide in which the flexible line is engaged, fastening being effected in the lower mouth zone of the end section of the tube guide.
Tube guide, in the sense of the invention, is understood equally well to be a vertical tubular element (I tube) as a progressive deviation tubular element (J tube). The function of the tube guide may, as a variant, be fulfilled by a lattice, or any other form of framework having an internal passage for guiding the flexible line.
The technique consisting in pulling a flexible line via one end through a tube guide secured to a structure, called the pull-in technique is commonly used in the offshore oil field.
This technique may be used in various configurations of exploitation systems of subsea oil fields and for example in a production installation comprising one or more pipes or flow-lines connecting the various subsea well-heads, or other equipment arranged on the seabed, to a structure forming a surface support such as described hereinabove or to a subsurface structure located at an intermediate level between the surface and the subsea bed, the said structure comprising various equipment and, particularly, pipework elements which must be connected via the flexible pipe to the equipment arranged on the seabed. Alternatively, the production installation may comprise, instead of equipment arranged on the seabed, a second surface support, or subsurface support, to which is connected the second end of the flexible pipe which is either entirely suspended as a chain between the two supports, or partially rests on the seabed.
In a first implementation of this type of installation, a flexible pipe is unrolled from a laying ship, a first end of the pipe being pulled by the pull-in technique so as to be able to connect this first end of the flexible pipe on board the surface support opposite which the laying ship is positioned.
The flexible pipe is then suspended as a chain between the laying ship and the surface support. Next, the laying ship moves away and starts to unroll the flexible pipe onto the seabed.
In a second implementation, once the flexible pipe has been laid over practically its entire length by the laying ship on the subsea bed, the second end of the flexible pipe is connected via a pulling cable to the surface support then pulled aboard and connected.
The connection between the terminal fitting of the flexible pipe and the network of pipework installed on the structure forming the support is usually performed above the surface of the water, at the level of a working bridge for example. Such a device has the advantage of facilitating the connection of the flexible pipe on board the platform. In contrast, despite the possibility for arranging, around the upper end of the flexible pipe a curvature limiter in the form of a stiffener securely fastened with respect to the support structure, this device exhibits relatively serious risks for the flexible tubular pipe which, devoid of protection, must pass through the splash zone.
In addition, when the support structure is floating, anchored, the flexible tubular pipe undergoes the consequences of excursion of the structure to which it is to be fixed, particularly due to the fact that the latter, pulling on its anchorages, moves away from its theoretical resting place with respect to the vertical.
According to another known device, in particular when the structure is an anchored floating platform, the connection between the flexible pipe and the platform is produced below the water surface, preferably in the bottom part of the platform in the vicinity of the keel. The advantage of such a device is that it puts the flexible pipe in relative shelter, moving it away from the zone of maximum turbulence created at the surface by the swell. It is, nevertheless, commonly considered necessary to install, in line with the connection of the flexible line to the structure, at the lower end of the tube guide secured to the structure, a curvature limiter in the form of a stiffener, securely fastened, and aiming to distribute the bending stresses and to prevent the radius of curvature imparted on the flexible line from attaining a critical lower value below the value at which the flexible line risks rapidly being put out of service. Such a device has the obvious drawback of requiring difficult and costly operations dependent upon weather conditions, in order to produce the subsea connections.
Moreover, U.S. Pat. No. 4,808,034 describes a device for connecting a flexible pipe to a floating structure in which an annular guide plate is connected to a stiffener mounted with clearance about the flexible pipe. The guide plate comprises, projecting from its front end, fixing pieces interacting with corresponding recessed pieces produced at the lower end of the vertical tube guide fixed to the structure.
In this embodiment, essentially two pulling phases are implemented with the aid of various lifting cables.
First of all, cables are fixed to the fixing pieces arranged projecting at the periphery of the guide plate and the assembly constituted by the guide plate secured to the stiffener and the pipe equipped with its fitting are pulled upwards, this said fitting finding itself bearing on the shoulder made up by the inner edge of the guide plate. Another lifting cable arranged axially and fixed to the fitting is brought up simultaneously.
Once the guide plate has been secured to the end of the tube guide, the said other lifting cable, fixed to the fitting, is used to pull the latter as well as the flexible line, through the tube guide, the fitting then being connected to the structure in a known manner at, or beyond the upper end of the tube guide.
This known device requires use of a plurality of pulling cables during various stages.
During the first phase of the operation for installing the device, it is necessary to haul several cables at once, balancing their respective lengths and tensions in order to pull on the guide plate which entrains the fitting and the flexible pipe and, furthermore, it is necessary simultaneously to bring the axial lifting cable with a minimum pulling force just sufficient for preventing this cable from having any slack.
This device is very difficult to implement, in particular in the case of flexible lines a certain length of which rests on the seabed, in order to carry out the pulling of flexible lines of long length and high weight, to which weight is added the weight of the fitting, of the stiffener and of the guide plate. In addition, the device described in the prior document is, bearing in mind the configuration of the means provided for the relative positioning and fixing of the guide plate and of the end of the tube guide, limited to an implementation with a vertical tube guide. Furthermore, this device presents the serious drawback that the fastening of the stiffener, such as produced by fixing the guide plate onto the end of the tube guide, lacks security.
The present invention proposes to produce a device which is easier to implement than the previously known devices, and is not limited to an implementation with a vertical tube guide, whilst allowing an effective and reliable fastening of the curvature limiter, and particularly under the water and preferably without the intervention of divers.